I. Field
The present disclosure relates generally to circuits, and more specifically to active circuits such as amplifiers and mixers.
II. Background
Amplifiers are commonly used in various electronics devices to provide signal amplification. Different types of amplifiers are available for different uses. For example, a wireless device such as a cellular phone may include a transmitter and a receiver for bidirectional communication. The transmitter may utilize a power amplifier (PA), the receiver may utilize a low noise amplifier (LNA), and the transmitter and receiver may utilize variable gain amplifiers (VGAs).
An LNA is commonly used in a receiver to amplify a low-amplitude signal received via a communication channel. The LNA is often the first active circuit encountered by the received signal and hence has a large impact on the performance of the receiver in several key areas. First, the LNA has a large influence on noise performance since the noise of the LNA is injected directly into the received signal and the noise of subsequent stages is effectively reduced by the gain of the LNA. Second, the linearity of the LNA has a large influence on both the design of subsequent stages in the receiver and receiver performance.
The LNA input signal typically includes various undesired signal components that may come from external interfering sources and leakage from a co-located transmitter. Nonlinearity in the LNA causes the undesired signal components to mix and generate cross modulation distortion that may fall within the desired signal bandwidth. The amplitude of the cross modulation distortion is determined by the amount of nonlinearity in the LNA. Cross modulation distortion components that fall within the desired signal bandwidth act as noise that degrades the signal-to-noise ratio (SNR) of the desired signal. The degradation in SNR caused by LNA nonlinearity may degrade the receiver performance and/or impact the design of subsequent stages. For example, more stringent requirements may be placed on the subsequent stages in order to meet the overall SNR specification of the receiver. A more linear LNA may relax the performance requirements of the subsequent stages.
There is therefore a need in the art for amplifiers having good linearity.